The issue with Blackwald Pixie is that you need 7 Mana to make full use of its effect. If you want to hold on to a card for that specific purpose it better be game-changing. A hero power needs to be VERY impactful to be worth it; like summoning 2 more Horsemen to win the game, or deal 16 damage in old Quest Warrior, or dealing 6 with Odd Hunter.
As it was already said, some of the new quests simply can't make use of it, and I think most of the others don't "need" it. Maybe in some it's worth considering, like in Warlock perhaps, since the hero power by itself is negating the mana investment. Still, I think the potential is rather limited. It wouldn't have high priority for me to include it in a deck.